The present invention relates to highly attenuated fibers comprising starch and polymers, processes of making the fibers, and specific configurations of the fibers, including microfibrils. The fibers are used to make nonwoven webs and disposable articles.
There have been many attempts to make nonwoven articles. However, because of costs, the difficultly in processing, and end-use properties, there are only a limited number of options. Useful fibers for nonwoven articles are difficult to produce and pose additional challenges compared to films and laminates. This is because the material and processing characteristics for fibers is much more stringent than for producing films, blow-molding articles, and injection-molding articles. For the production of fibers, the processing time during structure formation is typically much shorter and flow characteristics are more demanding on the material""s physical and rheological characteristics. The local strain rate and shear rate are much greater in fiber production than other processes. Additionally, a homogeneous composition is required for fiber spinning. For spinning very fine fibers, small defects, slight inconsistencies, or non-homogeneity in the melt are not acceptable for a commercially viable process. The more attenuated the fibers, the more critical the processing conditions and selection of materials.
Attempts have been made to process natural starch on standard equipment and existing technology known in the plastic industry. Fibers comprising starch are desired as the starch is environmentally degradable. Since natural starch generally has a granular structure, it needs to be xe2x80x9cdestructurizedxe2x80x9d before it can be melt processed into fine denier filaments. Modified starch (alone or as the major component of a blend) has been found to have poor melt extensibility resulting in difficulty in successfully production of fibers, films, foams or the like. Additionally, starch fibers are difficult to spin and are virtually unusable to make nonwovens due to the low tensile strength, stickiness, and the inability to be bonded to form nonwovens.
To produce fibers that have more acceptable processability and end-use properties, thermoplastic polymers need to be combined with starch. Selection of a suitable polymer that is acceptable for blending with starch is challenging. The polymer must have good spinning properties and a suitable melting temperature. The melting temperature must be high enough for end-use stability to prevent melting or structural deformation, but not too high of a melting temperature to be able to be processable with starch without burning the starch. These requirements make selection of a thermoplastic polymer to produce starch-containing fibers very difficult.
Consequently, there is a need for a cost-effective and easily processable composition made of natural starches and thermoplastic polymers. Moreover, the starch and polymer composition should be suitable for use in conventional processing equipment. There is also a need for disposable nonwoven articles made from these fibers.
The present invention is directed to highly attenuated fibers produced by melt spinning a composition comprising destructurized starch, a thermoplastic polymer, and a plasticizer. The present invention is also directed to highly attenuated fibers containing thermoplastic polymer microfibrils which are formed within the starch matrix of the fiber. The present invention is also directed to nonwoven webs and disposable articles comprising the highly attenuated fibers.